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Abstract

Swing, the sideways deviation of the cricket ball in flight, is a phenomenon which can be explained in terms of fluid dynamics. Conventional swing has been used since the beginning of the twentieth century and is effective with a new or well-preserved ball. A controversial form of swing emerged in Pakistan in the 1980s, featuring an older worn ball which, at a high speed, swung in the reverse direction. A cricket ball is asymmetric because of the presence of a seam, which is made up of rows of prominent encircling stitches. For conventional swing, this seam trips into turbulence the boundary layer adjacent to one hemisphere of the ball which remains attached to a greater angle (about 120) than does that on the other side (about 80) where no seam is present to trip the laminar boundary layer. The result is asymmetrical separation, leading to a skewed wake and a net pressure force on the ball perpendicular to the flight trajectory. Reverse swing is thought to be a consequence of the fact that asymmetry inverts at a high speed if the seam thickens the turbulent boundary layer on one side of the ball. Although the fluid dynamics causes of both conventional and reverse swing are well known, this paper demonstrates clearly, by means of flow visualization and pressure measurements, the inversion of this pressure asymmetry at Reynolds numbers greater than 170 x 10^3.